KHARTOUM (Reuters) - Insurgents shelled the main city of Sudan's oil-producing South Kordofan state near the border with South Sudan on Monday, both sides said, their first assault on the government stronghold since last year.

Fighting in the state has forced hundreds of thousands of people to flee their homes and stoked tensions between Sudan and South Sudan since they split apart last year under a 2005 peace deal that ended decades of civil war.

Sudan's army spokesman Al-Sawarmi Khalid said the insurgents had fired eight shells from about six kilometers outside Kadugli, the state capital, on Monday morning, killing one woman and wounding three other people.

He accused the insurgents of targeting a conference of political leaders scheduled to be held in the city. Presidential assistant Nafie Ali Nafie, a high-ranking official, had been scheduled to speak, state media reported.

"The infiltrators were targeting the Kadugli consultative conference ...but they failed to achieve that aim," Khalid said.

Rebel spokesman Arnu Lodi confirmed the rebels were shelling the town. He said the bombardments were in retaliation for attacks on their positions, not an attempt to derail the conference.

"We have to fight back and of course to carry out our objective of removing the government," he said.

Khartoum accuses South Sudan of backing the rebels - known as the Sudan People's Liberation Movement-North (SPLM-N) - who fought as part of the southern rebel army during the war. South Sudan denies the charge.

SHELLS LAND IN COMPOUND

Ray Torres, officer in charge at UNICEF, which has a compound in Kadugli, said the shelling began around 1140 and stopped around 1215.

One or two shells landed inside the UNICEF compound but did not explode, he said, adding that one women had been injured by a rocket explosion outside the compound.

Fighting broke out in South Kordofan in June last year and spread to Blue Nile, another border state, in September. Fighting has forced more than 600,000 people to flee their homes in both states.

The assault on Kadugli was the first the rebels have staged on the state capital since 2011, Lodi said.

Under international pressure, Sudan and South Sudan agreed last month to secure their border after clashing along it multiple times in the past year. Indirect talks between Khartoum and the SPLM-N have made no visible progress.

The SPLM-N last year formed an alliance with other rebels in the western Darfur region to overthrow the government of President Omar Hassan al-Bashir, but it is unclear how closely the insurgent groups have been coordinating since then.

(Reporting by Khalid Abdelaziz, Ulf Laessing and Alexander Dziadosz; Editing by Rosalind Russell)